Separation anxiety can be a very destructive behavioral trait. It is caused by the dog's frustration and fear. While most people believe that separation anxiety is a dog's inability to handle separation from its owner, the reality is that the dog wouldn't have behaved this way right from the beginning.
Why should we think it's the dog's fault and then just get rid of it. You are only passing on your problems to someone else. It is extremely unlikely that the new owner has the experience to fix separation anxiety in dogs you created.
The dog will get rehomed again until the owner no longer wants it. We all know what happens next. Separation anxiety can be prevented and retrained so that neither the owner nor the dog has to suffer.
You must change your routine to change the behavior. The dog will be fine when the owner goes to sleep. This allows him to have some space and can learn how to cope.
As he chews through the leads and is destructive when separated, the owner has to place him in a secure area where he can vent his anger without causing any harm or causing damage. The owner can now go to a different area, which will allow the dog to resume his routine.
The owner should remain out of reach and praise the dog when he calms down. This will help the dog associate calm and relaxation with separation. It will take some time for this to become routine, but it will eventually.